The recent increase in the use of portable electronic devices such as mobile telephones and notebook computers and the emerging trend of using rechargeable batteries in hybrid electric vehicles has created a need for smaller, lighter, longer lasting rechargeable batteries to provide the power to devices such as these. During the 1990s, lithium rechargeable batteries, specifically lithium-ion batteries, became popular and, in terms of units sold, now dominate the portable electronics marketplace and are set to be applied to new, cost sensitive applications. However, as more and more power hungry functions are added to the above mentioned devices (e.g. cameras on mobile phones), improved and lower cost batteries that store more energy per unit mass and per unit volume are required.
The basic composition of a conventional lithium-ion rechargeable battery cell including a graphite-based anode electrode is shown in FIG. 1. The battery cell includes a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector for the negative electrode (or anode) 10 and an aluminium current collector for the positive electrode (or cathode) 12 which are both externally connectable to a load or to a recharging source as appropriate. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16. A liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16. The polymer electrolyte material can be a solid polymer electrolyte or a gel-type polymer electrolyte and can incorporate a separator. The electrodes are referred to as an anode or cathode based upon their function during discharge of the cell, when current is supplied through a load. This means that the negative electrode is referred to as the anode and the positive electrode is referred to as the cathode. However, as known in the art, in a rechargeable cell each electrode can function as both an anode and a cathode, depending on whether the cell is being charged or discharged.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide cathode layer 16 via the electrolyte into the graphite-based anode layer 14 where it reacts with the graphite to create the compound, LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g. It will be noted that the terms “anode” and “cathode” are used in the sense that the battery is placed across a load.
It is well known that silicon can be used as the active anode material of a rechargeable lithium-ion electrochemical battery cell (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite. Crystalline silicon, when converted to the compound Li22Si5 by reaction with lithium in an electrochemical cell, has a maximum theoretical capacity of between 4000 and 4,200 mAh/g, considerably higher than the maximum capacity for graphite. Thus, if graphite can be replaced by silicon in a lithium rechargeable battery the desired increase in stored energy per unit mass and per unit volume can be achieved.
Many existing approaches of using a silicon or silicon-based active anode material in a lithium-ion electrochemical cell, however, have failed to show sustained capacity over the required number of charge/discharge cycles and are thus not commercially viable.
One approach disclosed in the art uses silicon in the form of a powder having particles with a diameter of 10 μm in some instances made into a composite with or without an electronic additive and containing an appropriate binder such as polyvinylidene difluoride; this anode material is coated onto a copper current collector. However, this electrode system fails to show sustained capacity when subjected to repeated charge/discharge cycles. It is believed that this capacity loss is due to partial mechanical isolation of the silicon powder mass arising from the volumetric expansion/contraction associated with lithium insertion/extraction to and from the host silicon. In turn this gives rise to electrical isolation of the silicon particles from both the copper current collector and each other. In addition, the volumetric expansion/contraction causes the individual particles to be broken up causing a loss of electrical contact within the spherical element itself.
Another approach known in the art designed to deal with the problem of the large volume changes during successive cycles is to make the size of the silicon particles that make up the silicon powder very small, i.e. in the 1-10 nm range. This strategy does not prevent the electrical isolation of the spherical elements from both the copper current collector and themselves as the silicon powder undergoes the volumetric expansion/contraction associated with lithium insertion/extraction. Importantly, the large surface area of the nano-sized elements can give rise to the creation of a lithium-containing surface film that introduces a large irreversible capacity into the lithium-ion battery cell. In addition, the large number of small silicon particles creates a large number of particle-to-particle contacts for a given mass of silicon and these each have a contact resistance and may thus cause the electrical resistance of the silicon mass to be too high. Furthermore, nano-sized particles tend to agglomerate into larger particles, making preparation of uniform electrode composites difficult.
The above problems have thus prevented silicon particles from becoming a commercially viable replacement for graphite in lithium rechargeable batteries and specifically lithium-ion batteries.
In another approach described by Ohara et al. in Journal of Power Sources 136 (2004) 303-306 silicon is evaporated onto a nickel foil current collector as a thin film and this structure is then used to form the anode of a lithium-ion cell. However, although this approach gives good capacity retention, this is only the case for very thin films (say ˜50 nm) and thus these electrode structures do not give usable amounts of capacity per unit area.
A review of nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells has been provided by Kasavajjula et al (J. Power Sources (2006), doi:10.1016/jpowsour.2006.09.84), herewith incorporated by reference herein.
Another approach described in UK Patent Application GB2395059A uses a silicon electrode comprising a regular or irregular array of silicon pillars fabricated on a silicon substrate. These structured silicon electrodes show good capacity retention when subjected to repeated charge/discharge cycles and this good capacity retention is considered by the present inventors to be due to the ability of the silicon pillars to absorb the volumetric expansion/contraction associated with lithium insertion/extraction from the host silicon without the pillars being broken up or destroyed. However, the structured silicon electrodes described in the above publication are fabricated using a high purity, single crystal silicon wafer and hence the electrode is expensive.
Selective etching of silicon-based materials to create silicon pillars is also known from U.S. Pat. No. 7,033,936. The pillars of this document are fabricated by depositing hemispherical islands of caesium chloride or silicon dioxide on a crystalline silicon substrate to form a mask surface, covering the substrate surface, including the islands, with a film, and removing the hemispherical structures (including the film covering them) from the surface to form a further mask having exposed areas where the hemispheres had been. The substrate is then etched in the exposed areas using reactive ion etching and the resist is removed, e.g. by physical sputtering, to leave an array of silicon pillars in the unetched regions, i.e. in the regions between the locations of the hemispheres attached to the silicon base.
An alternative chemical method for fabricating silicon pillars or nano-wires is described by Peng K-Q, Yan, Y-J, Gao S-P, and Zhu J., Adv. Materials, 14 (2002), 1164-1167, Adv. Functional Materials, (2003), 13, No 2 February, 127-132 and Adv. Materials, 16 (2004), 73-76. According to the method of Peng, et al. a single silicon wafer (which may be n- or p-type and has the {111} face exposed to solution) is etched at 50° C. using the following solution: 5M HF and 20 mM (0.02M) AgNO3. The mechanism postulated in these papers is that isolated nanoclusters of silver are electrolessly deposited on the silicon surface in an initial stage (nucleation). In a second (etching) stage, the silver nanoclusters and the areas of silicon surrounding them act as local electrodes that cause the electrolytic oxidation of the silicon in the areas surrounding the silver nanoclusters to form SiF6 cations, which diffuse away from the etching site to leave the silicon underlying the silver nanocluster in the form pillars.
K. Peng et al., Angew. Chem. Int. Ed., 44 (2005), 2737-2742; and K. Peng et al., Adv. Funct. Mater., 16 (2006), 387-394, describe a method of etching a single silicon wafer that is similar to that described in the earlier papers by Peng et al but the nucleation/silver nanoparticle deposition step and the etching step are performed in different solutions. In a first (nucleation) step, a silicon chip is placed in a solution of 4.6M HF and 0.01M AgNO3 for 1 minute. A second (etching) step is then performed in a different solution, namely 4.6M HF and 0.135M Fe(NO3)3 for 30 or 50 minutes. Both steps are carried out at 50° C. In these papers, a different mechanism is proposed for the etching step as compared to the earlier papers, namely that silicon underlying the silver (Ag) nanoparticles are removed and the nanoparticles gradually sink into the bulk silicon, leaving columns of silicon in the areas that are not directly underlying the silver nanoparticles.
In order to increase the uniformity and density of the pillars grown on silicon wafers and the speed of growth, it has been proposed in WO2007/083152 to conduct the process in the presence of an alcohol.
Indeed, Gamido et al, J. Electrochem. Soc. 143(12) 1996 describes the superior behaviour of HF/ethanol mixtures in the etching of silicon substrates.
It will be appreciated that each of these documents referred to above discloses methods of fabricating silicon pillars or fibres on silicon wafers or chips. Wafers or chips are generally expensive to make, which means that any fibres or wires fabricated there from have a high intrinsic cost.
WO2009/010758 discloses the etching of silicon powder instead of wafers, in order to make silicon material for use in lithium ion batteries. The resulting etched particles, an example of which is shown in FIG. 2, contain pillars on their surface and the whole of the resulting particles can be used as an anode material. Alternatively, the pillars can be severed from the particles to form silicon fibres and only the silicon fibres are used to make the anode. The etching method used is the same as that disclosed in WO2007/083152.
PCT/GB2009/002348 discloses a further method that can be used to fabricate silicon pillars from both highly pure and lower grade (for example, metallurgical grade) silicon materials such as particulate or granular silicon. The method involves treating the silicon material with a solution comprising 5 to 10M hydrofluoric acid (HF), 0.01 to 0.1M silver ions (Ag+), 0.02 to 0.2M nitrate ions (NO3−) and adding further nitrate ions to maintain the concentration of nitrate ions within the range specified during the treatment. The silicon particles are used in an amount in excess of 6 g of silicon per liter of etching solution.
Pillar arrays or detached silicon fibres have also been used in the fabrication of fuel cells, filters, sensors, field emitting diodes, chromatographic materials, solar capacitors, solar cells and electrical capacitors amongst other applications.
A problem with the methods disclosed in PCT/GB2009/002348 and WO2007/083152 and the other documents referred to herein above, is that the etching solution employs a high concentration of hydrofluoric acid (HF). Although a high fluoride concentration is believed to be essential for the etching step, a disadvantage of using such high concentrations of HF include the complications associated with recycling the excess HF left in the etchant solution after removal of the etched silicon material. Hydrogen fluoride is a highly corrosive material. The safety requirements associated with the handling of this material are complex and considerable. If the process involves the formation of insoluble salts of SiF62— the deposition of these salts may also contaminate the final product. The process of recycling the waste etchant is therefore both complex and costly. These cost implications have generally been ignored to date because of the belief in the field that a relatively high HF concentration is necessary if good quality pillars or fibres are to be produced. Indeed previous attempts to use lower concentrations of HF have resulted in very slow etch rates, wastage of the etching solution and poor quality silicon pillars or fibres.
The system parameters used to etch silicon-containing material such as silicon granules or powder have been found to be very different to those used for the etching of silicon wafers. Granules and powders have a much greater surface area than a silicon wafer of the same volume and tend to react more vigorously with an etching solution as a result. The rate of etching will, of course, depend upon the size and surface area of the silicon-containing particles being etched. It has been found, for example, that etching systems containing a high concentration of HF and a large quantity of silicon in the form of a granular or particulate material are liable to generate a considerable amount of heat and gas, which means that the system may be difficult to control and may result in an etched product containing silicon pillars that are fused together. Further, if the relative proportion of etching ingredients is incorrectly determined, excessive hydrogen gas may be generated and trapped at the surface of the silver nucleated silicon material thereby reducing access of the etching solution to the silicon surface and the extent to which the silicon surface can be etched. Finally it has been observed that if the HF concentration is too high, etching may proceed in both a vertical and a transverse direction, which may cause the pillars to become prematurely detached from the silicon surface. There is a need, therefore, for an etching system that can be used to efficiently etch the surface of a silicon powder or granule to give an etched silicon-containing product including on its surface an array of evenly distributed, well defined silicon-containing pillars having a uniform distribution of lengths and diameters.
There is a further need for an etching method, which reduces the safety, handling and cost issues associated with the use of etchant solutions comprising high concentrations of hydrogen fluoride but which is also able to produce silicon pillars or fibres of acceptable quality. The present invention addresses that problem.